In a disk drive, a magnetic recording head is made of read and write elements. The write element is used to record and erase data bits arranged in circular tracks on the disk while the read element plays back a recorded magnetic signal. The magnetic recording head is mounted on a slider which is connected to a suspension arm, the suspension arm urging the slider toward a magnetic storage disk. When the disk is rotated the slider flies above the surface of the disk on a cushion of air which is generated by the rotating disk.
The read element is generally made of a small stripe of multilayer magnetic thin films which have either magnetoresistance (MR) effect or giant magnetoresistance (GMR) effect, namely which changes resistance in response to a magnetic field change such as magnetic flux incursions (bits) from magnetic storage disk. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance in the read element and a corresponding change in the sensed current or voltage.
FIGS. 1 and 2A-2C illustrate examples of a conventional composite type thin-film magnetic head 10. FIG. 1 is a cross-sectional view of the head 10 perpendicular to the plane of the air bearing surface (ABS). FIG. 2A shows the slider 11 flying above the disk 13.
In these figures, the reference numeral 12 denotes a substrate, 15 denotes an undercoating, 20 denotes a lower shield layer of the MR reproducing head part, 21 denotes an upper shield layer of the MR head part, which can also act as a lower pole of an inductive recording head part, 22 denotes a MR layer provided through an insulating layer 23 between the lower shield layer 20 and the upper shield layer 21, 26 denotes a write gap layer, 27 denotes a lower insulating layer deposited on the upper shield layer 21, 28 denotes a coil conductor formed on the lower insulating layer 27, 29 denotes an upper insulating layer deposited so as to cover the coil conductor 28, 30 denotes an upper pole, and 34 denotes a pad that would connect the read or write coil to other components in the drive. In general, there would be a plurality of pads 34 on the slider 11. Note that the pad 34 connects directly to the coil conductor 28. The upper pole 30 is magnetically connected with the lower pole (upper shield layer) 21 at its rear portion so as to constitute a magnetic yoke together with the lower pole 21.
As recording density and data transfer rate have increased over the past a few years, critical dimensions in the recording device such as track width read and write gap and coil size have decreased accordingly. Also, the fly height between the air bearing surface (ABS) 32 and the media have become smaller and smaller. For reference, recording heads with 40 gb/in2 products typically have fly heights of about 12 nanometers. This fly height will continue to decrease in the future. This reduction in head critical dimensions and fly height, while beneficial to magnetic performance, also comes with cost on thermal and mechanic reliability.
The thermal expansion coefficients for the substrate and the various layers of the head differ, so when the head becomes heated during use, some layers will begin to protrude from the ABS. FIG. 2B depicts the head 10 when the write element is not operating, and particularly that the spacing may vary due to recession of various materials and structure due to the ABS fabrication process. FIG. 2C is a detailed diagram of the heat transfer and protrusion profile of the head 10 when the head is active (e.g., when the write coil is energized). One issue with heads is that the write-induced protrusion of the pole and overcoat can cause head-media contact, resulting in errors. This can affect the write head signal to noise ratio with alterations in the magnetic spacing between the head and the media. In older generations of heads, this was not a problem because the head was flying much higher and device size was bigger leading to easier heat dissipation. However, the coil length in modem heads has decreased to accommodate high data rate advancement. Consequently, ohmic heating from write current through coil and eddy current in write pole/yoke and magnetic hysteresis of magnetic materials are confined in a tiny space near the ABS, which typically lead to unacceptable thermal protrusion and drive reliability concerns. As can be seen in FIG. 2C, the top write pole 30 and overcoat protrude from the ABS 32 toward the media 13. The protrusion amount can be 1-4 nanometers.
The thermal expansion is proportional to the temperature, so it would be desirable to reduce the temperature in order to limit the thermal expansion. This in turn would reduce protrusion.
There are several factors that limit the reduction in slider flying height. These factors might reasonably be ignored at flying heights of above 20 nanometers, but would become major concerns at flying heights on the order of <5 nanometers. These include variations in the sliders themselves, variations in the structure that supports the sliders, and media surface roughness.
More particularly, normal tolerances in slider fabrication lead to structural variations among the sliders in any given batch. Consequently, the flying heights of sliders in the batch are distributed over a range, although the flying height of each slider individually is substantially constant.
Variations in supporting structure occur primarily in the transducer support arm, the suspension or gimballing structure, slider geometry and load arm. These variations influence the flying height, and the nature of a given slider's reaction to any disturbances, e.g. due to shock or vibration.
Disk roughness also becomes more of a problem at lower slider flying heights. With maximum peaks more likely to protrude into a normal range of slider operation, the probability of unintended and damaging slider/disk contact increases. The risk of damage from these discontinuities is greater at lower slider flying heights.
Thermal effects also are exaggerated by minute slider flying heights. Thermal effects include the natural tendency of materials to expand when heated, quantified by a temperature coefficient of thermal expansion more conveniently called a thermal expansion coefficient. Materials with higher coefficients expand more in response to a given temperature increase. When materials having different thermal expansion coefficients are contiguous and integral, their differing expansion when heated leads to elastic deformations and elastic restoring forces in both of the materials. Reduced flying heights increase the need to take thermal expansion and thermally induced elastic deformation into account.
One proposed design of a slider would drag on the disk surface, thereby more precisely fixing a head/disk spacing based on a peak roughness of the disk surface. Any improvement in setting the transducer/recording surface gap, however, would be at the cost of excessive wear to the slider, media recording surface, or both.
What is needed is a way to allow the head to fly at a higher physical spacing when the head is not in a read or write mode, thereby protecting the head, yet be in close proximity to the media during reading and/or writing for allowing heads to read and write with reduced track width, bit length and error rate.